High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) is a technology that is capable of separating gas-phase ions at atmospheric pressure. In FAIMS, the ions are introduced into an analytical gap across which a radio frequency (rf) waveform, the magnitude of which is referred to as dispersion voltage (DV), is applied such that the ions are alternately subjected to high and low electric fields. The waveform is asymmetric; the high field is applied for one time unit followed by an opposite-polarity low field of half the high field component that is applied for twice as long. The field-dependent change in the mobility of the ions causes the ions to drift toward the walls of the analytical gap. Since the dependence of ion mobility on electric field strength is compound specific, this leads to a separation of the different types of ions one from the other, and is referred to as the FAIMS separation or the FAIMS mechanism. In order to transmit an ion of interest through FAIMS, an appropriate direct current compensation voltage (CV) is applied to compensate for the drift of the ion of interest toward the analyzer wall. By varying the CV, different ions are selectably transmitted through the FAIMS device.
Different FAIMS electrode geometries are known in the art. One specific type of electrode geometry, which is referred to as the “side-to-side” FAIMS geometry, includes typically a set of overlapping inner and outer electrodes. In particular, the inner electrode often is provided in the form of a circularly cylindrical rod-shaped electrode, whilst the outer electrode has a similarly curved inner surface that is spaced-apart from and facing the inner electrode. The annular space between the inner electrode and outer electrode defines an analytical gap for separating different types of ions one from another, according to the above-mentioned FAIMS mechanism. Ions are produced at an ionization source, such as for instance an electrospray ionization (ESI) source, and are introduced into the analytical gap via one or more ion inlet orifices. Once inside the analytical gap, the ions travel circumferentially in both directions around the inner electrode toward an ion outlet orifice. Some types of ions do not have stable trajectories under the selected combination of CV and DV and are lost due to collisions with an electrode surface, whilst other types of ions are carried to the ion outlet orifice and then out of the analytical gap for subsequent analysis or collection.
A feature that is common to all current side-to-side FAIMS devices, as well as FAIMS devices that are based on some other common electrode geometries, is that at least one ion inlet orifice is defined through the outer electrode in such a way that ion introduction is opposed directly by the electrical field within the analytical gap during one portion of the asymmetric waveform cycle. In fact, the electrical field extends into the ion inlet orifice and accordingly the electrical field begins to influence ion motion even before the ions actually enter the analytical gap. The result is that within the ion inlet orifice, and immediately after the ions enter the analytical gap, the ion trajectories oscillate first directly toward the inner electrode during application of one portion of the asymmetric waveform and then directly away from the inner electrode during application of another portion of the asymmetric waveform. Thus, the ions tend to “jitter” in and out of the analytical gap during introduction, although the net motion is still toward the inner electrode since the ions are also entrained in a flow of a carrier gas. Once inside the analytical gap, the carrier gas flow splits and carries the ions in both directions around the inner electrode. The electrical field continues to induce the same oscillations in the ion trajectories, and only those ions for which the oscillations are compensated by the compensation voltage actually reach the ion outlet orifice.
The above-mentioned “jitter” motion that occurs during ion introduction has a tendency to increase the width of the ion injection window as well as to decrease the ion introduction efficiency. Since one of the advantages of the side-to-side FAIMS device is the short ion flow path length around the inner electrode, and consequently a relatively short ion transit time through the analytical gap, it will be apparent that a longer ion injection window has an adverse effect on the performance of a side-to-side FAIMS device. Accordingly ion inlet configurations, such as those described previously by Guevremont et al. in U.S. Pat. No. 6,753,522 and including three or more separate ion inlet orifices that are arranged in rows or other geometrical arrangements, tend not to result in optimal performance. In particular, each ion inlet configuration disclosed by Guevremont et al. includes at least one ion inlet orifice that is defined through the outer electrode in such a way that ion introduction is opposed directly by the electrical field within the analytical gap during one portion of the asymmetric waveform cycle. This is particularly problematic when the side-to-side FAIMS device is being used to separate or analyze ions on a very short time scale. One such example involves analysis of ions that are generated from samples that are eluting from a high-performance liquid chromatography (HPLC) apparatus, or from another similar chromatographic apparatus.
Of course, the same “jitter” motion also occurs when ions are introduced into FAIMS devices that are based on other electrode geometries. Of particular note is the so-called domed-FAIMS (d-FAIMS) electrode geometry. In a d-FAIMS device, ions enter into an analytical gap between two concentric cylindrical electrodes and spread out in a ring-shaped cloud of finite thickness at a particular radial distance between the two electrodes. The ions travel along the length of the device and are directed radially inward around a domed surface terminus of the inner electrode prior to being extracted via an ion outlet orifice. Since the ions are introduced via an ion inlet in such a way that ion introduction is opposed directly by the electrical field within the analytical gap during one portion of the asymmetric waveform cycle, the d-FAIMS device is expected to show behavior similar to that which has been described above.
Accordingly, there exists a need for a FAIMS cell that overcomes at least some of the above-mentioned limitations.